The invention relates to fuel cells.
A fuel cell can convert chemical energy to electrical energy by promoting a chemical reaction between two gases.
One type of fuel cell includes a cathode flow field plate, an anode flow field plate, a membrane electrode assembly disposed between the cathode flow field plate and the anode flow field plate, and two gas diffusion layers disposed between the cathode flow field plate and the anode flow field plate. A fuel cell can also include one or more coolant flow field plates disposed adjacent the exterior of the anode flow field plate and/or the exterior of the cathode flow field plate.
Each flow field plate has an inlet region, an outlet region and open-faced channels connecting the inlet region to the outlet region and providing a way for distributing the gases to the membrane electrode assembly.
The membrane electrode assembly usually includes a solid electrolyte (e.g., a proton exchange membrane, commonly abbreviated as a PEM) between a first catalyst and a second catalyst. One gas diffusion layer is between the first catalyst and the anode flow field plate, and the other gas diffusion layer is between the second catalyst and the cathode flow field plate.
During operation of the fuel cell, one of the gases (the anode gas) enters the anode flow field plate at the inlet region of the anode flow field plate and flows through the channels of the anode flow field plate toward the outlet region of the anode flow field plate. The other gas (the cathode gas) enters the cathode flow field plate at the inlet region of the cathode flow field plate and flows through the channels of the cathode flow field plate toward the cathode flow field plate outlet region.
As the anode gas flows through the channels of the anode flow field plate, the anode gas passes through the anode gas diffusion layer and interacts with the anode catalyst. Similarly, as the cathode gas flows through the channels of the cathode flow field plate, the cathode gas passes through the cathode gas diffusion layer and interacts with the cathode catalyst.
The anode catalyst interacts with the anode gas to catalyze the conversion of the anode gas to reaction intermediates. The reaction intermediates include ions and electrons. The cathode catalyst interacts with the cathode gas and the reaction intermediates to catalyze the conversion of the cathode gas to the chemical product of the fuel cell reaction.
The chemical product of the fuel cell reaction flows through a gas diffusion layer to the channels of a flow field plate (e.g., the cathode flow field plate). The chemical product then flows along the channels of the flow field plate toward the outlet region of the flow field plate.
The electrolyte provides a barrier to the flow of the electrons and gases from one side of the membrane electrode assembly to the other side of the membrane electrode assembly. However, the electrolyte allows ionic reaction intermediates to flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly.
Therefore, the ionic reaction intermediates can flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly without exiting the fuel cell. In contrast, the electrons flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly by electrically connecting an external load between the anode flow field plate and the cathode flow field plate. The external load allows the electrons to flow from the anode side of the membrane electrode assembly, through the anode flow field plate, through the load and to the cathode flow field plate.
Electrons are formed at the anode side of the membrane electrode assembly, indicating that the anode gas undergoes oxidation during the fuel cell reaction. Electrons are consumed at the cathode side of the membrane electrode assembly, indicating that the cathode gas undergoes reduction during the fuel cell reaction.
For example, when hydrogen and oxygen are the gases used in a fuel cell, the hydrogen flows through the anode flow field plate and undergoes oxidation. The oxygen flows through the cathode flow field plate and undergoes reduction. The specific reactions that occur in the fuel cell are represented in equations 1-3.
H2xe2x86x922H++2exe2x88x92xe2x80x83xe2x80x83(1) 
xc2xdO2+2H++2exe2x88x92xe2x86x92H2O xe2x80x83xe2x80x83(2) 
H2+xc2xdO2xe2x86x92H2O xe2x80x83xe2x80x83(3) 
As shown in equation 1, the hydrogen forms protons (H+) and electrons. The protons flow through the electrolyte to the cathode side of the membrane electrode assembly, and the electrons flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly through the external load. As shown in equation 2, the electrons and protons react with the oxygen to form water. Equation 3 shows the overall fuel cell reaction.
In addition to forming chemical products, the fuel cell reaction produces heat. One or more coolant flow field plates are typically used to conduct the heat away from the fuel cell and prevent it from overheating.
Each coolant flow field plate has an inlet region, an outlet region and channels that provide fluid communication between the coolant flow field plate inlet region and the coolant flow field plate outlet region. A coolant (e.g., liquid de-ionized water) at a relatively low temperature enters the coolant flow field plate at the inlet region, flows through the channels of the coolant flow field plate toward the outlet region of the coolant flow field plate, and exits the coolant flow field plate at the outlet region of the coolant flow field plate. As the coolant flows through the channels of the coolant flow field plate, the coolant absorbs heat formed in the fuel cell. When the coolant exits the coolant flow field plate, the heat absorbed by the coolant is removed from the fuel cell.
To increase the electrical energy available, a plurality of fuel cells can be arranged in series to form a fuel cell stack. In a fuel cell stack, one side of a flow field plate functions as the anode flow field plate for one fuel cell while the opposite side of the flow field plate functions as the cathode flow field plate in another fuel cell. This arrangement may be referred to as a bipolar plate. The stack may also include monopolar plates such as, for example, an anode coolant flow field plate having one side that serves as an anode flow field plate and another side that serves as a coolant flow field plate. As an example, the open-faced coolant channels of an anode coolant flow field plate and a cathode coolant flow field plate may be mated to form collective coolant channels to cool the adjacent flow field plates forming fuel cells.
The invention relates to fuel cells.
Under some operating conditions, such as those occurring at high operating levels and/or at relatively high temperatures, the performance of a fuel cell or a fuel cell stack can be reduced, for example, compared to when the fuel cell or fuel cell stack is operating at relatively lower temperatures and/or operating levels. Without wishing to be bound by theory, it is believed that this decreased performance can be caused by degradation of the electrolyte by peroxide. It is believed that peroxide can be produced from the chemical reaction occurring at the cathode, e.g., as an intermediate; and/or peroxide can be produced as a result of protons and oxygen diffusing through the electrolyte. The peroxide is capable of reacting with and degrading the electrolyte. For example, in embodiments in which the electrolyte includes a fluorocarbon polymer, the peroxide can be catalyzed by impurities, e.g., iron, in the electrolyte, and hydrogen fluoride (a product of a degradation reaction) can be detected in a gas stream exiting the fuel cell stack. At relatively high temperatures, the degree of degradation is further enhanced. In some cases, for about every 10xc2x0 C. increase in operating temperature, the degree of electrolyte degradation can double.
Decreased performance of the fuel cell or the fuel cell stack can also be caused by condensation of water carried by a reactant gas, which can be saturated with water. Condensed water can act a gas diffusion barrier, e.g., by resisting the flow of oxygen gas. As a result, this can lower the performance of the fuel cell by preventing the fuel cell reactions from occurring.
In one aspect, the invention features a fuel cell or a fuel cell stack having good resistance to membrane degradation and/or good handling of condensed water, e.g., good dispersion or low absorption of water. In some embodiments, the fuel cell or the fuel cell stack minimizes peroxide. In certain embodiments, the fuel cell or the fuel cell stack allows condensed water to be effectively removed from the cell or stack. As a result, the performance of the fuel cell or the fuel cell stack can be enhanced, e.g., at high operating levels. In embodiments, the fuel cell or the fuel cell stack includes one or more non-electrolytic layers adjacent to one or more catalyst layers. The non-electrolytic layer can have a relatively compact and economical design.
In another aspect, the invention features a fuel cell including a first gas diffusion layer, a second gas diffusion layer, an electrolyte between the first and the second gas diffusion layers, a first catalyst layer between the electrolyte and the first gas diffusion layer, a first non-electrolytic layer between the first catalyst layer and the first gas diffusion layer, and a second catalyst layer between the electrolyte and the second gas diffusion layer.
Embodiments may include one or more of the following features. The first nonelectrolytic layer includes a non-electrolytic polymer, such as a fluorine-containing resin, e.g., polytetrafluoroethylene. The first non-electrolytic layer includes a copolymer of tetrafluoroethylene and hexafluoropropylene. The first non-electrolytic layer includes electrically conductive particulate material, such as platinum, e.g., unsupported on another material. The first catalyst layer is a cathode or an anode. The electrolyte includes a proton exchange membrane. The first non-electrolytic layer is discrete from the first catalyst layer.
The fuel cell can further include a second non-electrolytic layer between the second catalyst layer and the second gas diffusion layer.
The fuel cell can further include a first flow plate, and a second flow plate, wherein the first gas diffusion layer is between the first non-electrolytic layer and the first flow plate, and the second gas diffusion layer is between the second catalyst layer and the second flow plate.
The first non-electrolytic layer can have a thickness substantially equal to the thickness of the first catalyst layer.
In another aspect, the invention features a fuel cell including a first gas diffusion layer, a second gas diffusion layer, an electrolyte between the first and the second gas diffusion layers, a first catalyst layer between the electrolyte and the first gas diffusion layer, the first catalyst layer comprising a first electrolytic polymer and a first non-electrolytic polymer, and a second catalyst layer between the electrolyte and the second gas diffusion layer.
Embodiments may include one or more of the following features. The first electrolytic polymer includes an ionomer, such as a sulphonated fluorocarbon polymer. The first non-electrolytic polymer includes a fluorine-containing resin, such as polytetrafluoroethylene. The first non-electrolytic polymer includes a copolymer of tetrafluoroethylene and hexafluoropropylene. The first electrolytic polymer and the first non-electrolytic polymer form a mixture.
The second catalyst layer can include a second electrolytic polymer and a second non-electrolytic polymer.
In another aspect, the invention features a method of operating a fuel cell system. The method includes contacting a first gas with a first non-electrolytic layer contained in a first fuel cell, and contacting the first gas with a first catalyst layer contained in the first fuel cell.
Embodiments may include one or more of the following features. The first gas can include a cathode gas, e.g., having oxygen. The first gas can contact the first non-electrolytic layer before the first gas contacts the first catalyst layer. The method further includes contacting a second gas with a second non-electrolytic layer contained in the first fuel cell, and contacting the second gas with a second catalyst layer contained in the first fuel cell. The first non-electrolytic layer includes polytetrafluoroethylene and platinum.
In another aspect, the invention features a fuel cell including an electrolyte, a gas diffusion layer, a plurality of layers between the electrolyte and the gas diffusion layer, at least two of the layers having different concentrations of a non-electrolytic material, and an electrode layer between the electrolyte and the plurality of layers.
Embodiments may include one or more of the following features. The plurality of layers includes a layer adjacent to the gas diffusion layer having the highest concentration of non-electrolytic material relative to other layers of the plurality of layers. The plurality of layers includes a concentration gradient of the non-electrolytic material between the electrolyte and the gas diffusion layer. The concentration gradient is substantially linear. The concentration gradient decreases from the gas diffusion layer to the electrolyte. The non-electrolytic material includes polytetrafluoroethylene.
In another aspect, the invention features a fuel cell having an electrolyte, a gas diffusion layer, and a plurality of layers between the electrolyte and the gas diffusion layer. At least two of the plurality of layers include a non-electrolytic material and an electrolytic material, and at least two of the plurality of layers having different concentrations of the non-electrolytic material.
Embodiments may include one or more of the following features. The plurality of layers includes a layer adjacent to the gas diffusion layer having the highest concentration of the non-electrolytic material relative to the other layers of the plurality of layers. The plurality of layers includes a concentration gradient of the non-electrolytic material between the electrolyte and the gas diffusion layer. The concentration gradient of the non-electrolytic material is substantially linear. The concentration gradient of the non-electrolytic material decreases from the gas diffusion layer to the electrolyte. The plurality of layers includes a layer adjacent to the electrolyte having the highest concentration of electrolytic material relative to other layers of the plurality of layers. The plurality of layers includes a concentration gradient of the electrolytic material between the electrolyte and the gas diffusion layer. The concentration gradient of the electrolytic material is substantially linear. The concentration gradient of the electrolytic material increases from the gas diffusion layer to the electrolyte.
The plurality of layers can include a catalyst. The non-electrolytic material can include polytetrafluoroethylene.
Other features, aspects, and advantages of the invention will be apparent from the drawings, description, and claims.